Process for manufacturing a clad sheet which includes an abrasion-resistant layer made of tool steel, and clad sheet obtained

ABSTRACT

A block is produced by juxtaposition and joining of at least one plate made of tool steel and of at least one plate made from mild steel. The block is hot rolled and then it is cooled to a temperature below 200 DEG  C. The rolled block is tempered at a temperature above 450 DEG  C. after cooling. The clad sheet obtained includes a layer made of tool steel having a tempered martensitic structure and containing primary carbides and a fine dispersion of secondary carbides.

This application is a Division of application Ser. No. 08/047,434 filedon Apr. 15, 1993, now U.S. Pat. 5,908,710.

The invention relates to the manufacture of a clad sheet which includesan abrasion-resistant layer made of a steel of the tool-steel type and alayer made of an easily-weldable mild steel.

In mining, quarrying, civil engineering or other sectors of industry, itis known to use clad sheets having an abrasion-resistant layer, made oftool steel, and an easily weldable layer made of mild steel, in order toproduce equipment such as hopper casings, dozer blades or other devicessubjected to abrasion.

These clad sheets are generally obtained by rolling and are used in theas-rolled cooled state. These clad sheets have the maximum hardness thatit is possible to obtain and therefore a high abrasion resistance. Butit is desirable to improve this abrasion resistance further. Moreover,during the cooling, after rolling, the clad sheets form a bimetal anddistort. It is necessary to planish them. On account of the greathardness of the layer made of tool steel, this operation is difficultand generates large residual stresses in the clad sheets, stresses whichare annoying during implementation and in use.

The object of the present invention is to provide a process enablingclad sheets to be obtained having a layer made of tool steel, theabrasion resistance of which is improved and is free of residualstresses.

For this purpose, the subject of the invention is a process formanufacturing a clad sheet which includes an abrasion-resistant layermade of tool steel and an easily weldable layer made of mild steel,consisting in producing a block by juxtaposition and joining of at leastone plate made of tool steel and of at least one plate made of mildsteel containing less than 0.25% carbon and having an carbon equivalentof less than 0.50%, in hot rolling the block, then in cooling it to atemperature below 200° C. and in cutting it in order to obtain at leastone clad sheet, characterised in that, after cooling to a temperaturebelow 200° C. and before cutting the block, the hot-rolled blockundergoes a tempering treatment at a temperature above 450° C.

The block preferably is made up of two plates made of tool steel whichare gripped tightly between two plates made of mild steel.

The cooling after rolling may be an air cooling.

The tempering temperature lies between 450° C. and 650° C. andpreferably between 500° C. and 600° C.

The tool steel may have a chemical composition such that:

    ______________________________________                                                   0.5% ≦ C ≦                                                              2.5%                                                                  3% ≦ Cr ≦                                                               16%                                                                   Mo ≦                                                                           2%                                                                    V ≦                                                                            2%                                                                    W ≦                                                                            2%                                                                    Mn ≦                                                                           2%                                                                    Ni ≦                                                                           3%                                                                    Si ≦                                                                           2%                                                         ______________________________________                                    

and preferably:

1%≦C≦2%

and 8% ≦Cr≦13%.

The subject of the invention is also a clad sheet having improvedabrasion resistance which includes a layer made of tool steelcomprising, in its chemical composition, more than 0.5% C and more than3% Cr and a layer made of mild steel comprising, in its chemicalcomposition, less than 0.25% C and having a carbon equivalent of lessthan 0.50%, and the structure of the tool-steel layer of which is atempered martensite containing a fine dispersion of secondary carbides.The tool-steel layer preferably also contains coarse primary carbides.

The tool steel contains, in its chemical composition:

    ______________________________________                                                   0.5% ≦ C ≦                                                              2.5%                                                                  3% ≦ Cr ≦                                                               16%                                                                   Mo ≦                                                                           2%                                                                    V ≦                                                                            2%                                                                    W ≦                                                                            2%                                                                    Mn ≦                                                                           2%                                                                    Ni ≦                                                                           3%                                                                    Si ≦                                                                           2%                                                         ______________________________________                                    

and, more particularly:

1%≦C≦2%

and 8%≦Cr≦13%.

The clad sheets thus obtained are highly planar, are free of residualstresses, are easily weldable on one surface and have an improvedabrasion resistance on the other surface.

The invention will now be described in more detail, but in anon-limiting way.

The clad sheets according to the invention are made up of a layer madeof very hard tool steel clad on a layer of weldable mild steel calledthe base.

The class of tool steels is very broad and is principally characterisedby a carbon content greater than 0.5% and generally less than 2.5% and achromium content lying between 3% and 16%; this combination enables aself-hardening steel to be obtained having a very hard martensiticstructure and possibly containing carbides. These steels may alsocontain carbide-forming elements such as molybdenum, vanadium ortungsten in amounts generally less than 2%. They furthermore containmanganese (less than 2%) and silicon (less than 2%) and may contain upto 3% nickel. The carbon content preferably lies between 1% and 2% andchromium between 8% and 13%. The high chromium contents are desirable inorder to obtain primary carbides which precipitate upon solidificationand which are quite coarse (their dimension is of the order of 10 μm).

The mild steels used for producing the base are structural steelscontaining carbon, silicon, manganese and possibly chromium, molybdenum,nickel, vanadium, copper and impurities. In order to be easily weldable,the steels contain less than 0.25% carbon and have a carbon equivalentof: ##EQU1##

In order to produce a clad sheet according to the invention, a block isproduced made up of two plates or inserts made of tool steel which aregripped tightly between two plates or bases made of mild steel. Theassembly is fastened together and closed by a peripheral weld beadaccording to a technique known to the person skilled in the art.

The block is then heated above 1000° C. and hot rolled in order toobtain a rolled block, the dimensions of which correspond to thedimensions of the sheets which it is desired to obtain. The rolling iscompleted, for example, between approximately 800° C. and 900° C.

The rolled block is then left to air cool to a temperature in thevicinity of ambient temperature, for example 70° C. and in all casesless than 200° C. During this cooling, the plates made of tool steelundergo self-hardening and acquire a very hard martensitic structure;the layer made of mild steel acquires a structure of theferrite-pearlite type.

After cooling, the rolled block is tempered at a temperature lyingbetween 450° C. and 650° C. and preferably between 300° C. and 600° C.in order to relax the stresses induced in the clad layers, to soften themartensitic matrix and to cause secondary carbides to be precipitated.

The relaxation of the stresses, since this is performed in a closed andclamped block, is manifested by a planishing of the clad sheets.

This thermal self-planishing therefore has the result of correcting thenatural bimetallic-effect distortion without having to carry out theusual planishing operation on a mechanical planisher, this operationbeing dangerous on this type of hard product and generating in all caseshigh residual stresses.

The precipitation of fine secondary carbides of dimension of the orderof 1 μm makes it possible to limit the softening of the as-temperedmartensitic matrix.

The tempering temperature is chosen to be above 450° C. because, belowthis, the relaxation of the stresses is insufficient and theprecipitation of the secondary carbides is very low. It does not exceed650° C. because, above this, the martensitic structure becomes too softand because it has to remain below the austenitic-transformationtemperature.

After tempering, the rolled block is cut along its perimeter, whichenables two highly planar clad sheets to be obtained with have a layermade of weldable mild steel having a ferrite-pearlite structure and alayer made of very hard tool steel having a softened martensiticstructure containing a fine precipitation of secondary carbides and, ifthe carbon and chromium contents are sufficient, coarse primary carbideshaving precipitated upon solidifying.

Surprisingly, it has proved to be the case that these air-quenched andtempered tool steels had an abrasion resistance very markedly greaterthan that of the same as-quenched tool steels, which are a prioriharder. This is probably explained by the fine precipitation of chromiumcarbides.

By way of example, clad sheets were produced whose tool-steel layer wasmade up of a D2-type steel according to the American Standard AISI andhaving the composition:

    ______________________________________                                        C     Si      Mn       Ni    Cr     Mo    V                                   ______________________________________                                        1.5%  0.34%   0.35%    0.19% 11.5%  0.72% 0.92%                               ______________________________________                                    

The tempering temperature was 500° C. and the hardness of the layer madeof tool steel was 54 HRC.

Comparative tests were carried out between this clad sheet and a cladsheet whose tool-steel layer had the same composition but which had notundergone tempering (as-cooled state) by causing specimens to rotate inquartz granulates and by measuring the weight loss after a defined time.It has proved to be the case that the weight loss of the specimen havingundergone a tempering treatment was 30% less than the weight loss of theas-quenched specimen, that is a 30% increase in abrasion resistance.

We claim:
 1. A process for manufacturing a planar clad sheet having improved abrasion resistance an being substantially free of residual stresses, said planar clad sheet comprising a layer made of tool steel having a chemical composition comprising, by weight, more than 0.5% carbon and more than 3% chromium, and a layer made of low alloy steel having a chemical composition comprising, by weight, up to 0.25% carbon and a carbon equivalent up to 0.5%, the layer made of low alloy steel having a rolled structure and the layer made of tool steel having a rolled structure and comprising tempered martensite, coarse primary carbides and a fine dispersion of secondary carbides, said process comprising;producing a block by juxtaposing and joining at least one plate made of tool steel and at least one plate made of low alloy steel, hot rolling the block, cooling the block to a temperature below 200° C., tempering the block at a temperature above 450° C., and cutting the block in order to obtain said planar clad sheet.
 2. Process according to claim 1, wherein the block is made up of two plates made of tool steel which are gripped tightly between two low alloy steel plates.
 3. Process according to claim 1, wherein the cooling is a simple air cooling.
 4. Process according to claim 1, wherein the tempering temperature lies between 450° C. and 650° C.
 5. Process according to claim 4, wherein the tempering temperature lies between 500° C. and 600° C.
 6. Process according to claim 1, wherein the tool steel comprises, in its chemical composition:

    ______________________________________                                                    0.5% < C ≦                                                                      2.5%                                                                   3% ≦ Cr ≦                                                                16%                                                                    Mo ≦                                                                            2%                                                                     V ≦                                                                             2%                                                                     W ≦                                                                             2%                                                                     Mn ≦                                                                            2%                                                                     Ni ≦                                                                            3%                                                                     Si ≦                                                                            2%.                                                         ______________________________________                                    


7. Process according to claim 6, wherein the tool steel comprises, in its chemical composition:1%≦C≦2%; 8%≦Cr≦13%.
 8. The process as claimed in claim 1, wherein said coarse primary carbides have dimensions of approximately 10 μm and said secondary carbides have dimensions of approximately 1 μm.
 9. The process as claimed in claim 1, wherein said tool steel has a carbon content of from 1 to 2% by weight and a chromium content of from 8 to 13% by weight.
 10. The process as claimed in claim 1, wherein said tool steel layer has a hardness of 54 HRC and comprises the following elements in the indicated weight percentages:

    ______________________________________                                                 C            1.5                                                               Si           0.34                                                              Mn           0.35                                                              Ni           0.19                                                              Cr          11.5                                                               Mo           0.72                                                              V            0.92.- - -                                                ______________________________________                                     